It's not small, it adds quite some size to the 3DS. For me the added size and weight makes the 3DS a lot more comfortable to hold, feels really nice.
The added battery time is huge, I recently played SMT Devil Survivor Overclocked on my PP+-less US 3DS (my PP+ is on my EU 3DS) while in hospital and was going nuts because the battery was empty all the time, this doesn't come up with the Power Pak+ at all anymore.

Edit: Oh and yes Amazon.com does ship it to my adress. "Sold by: Amazon Export Sales, Inc." The $15 Power Pak+ costs a total of 36,66€ with shipping and taxes though, so I'd recommend to pool some stuff before ordering something there.

i love this battery pack. it gives more grip if you have bigger hands. and battery lasts a looong time, lot longer than the normal battery, i would recommend it. although there is a con about this. i dont think it will fit with the new slider extension, so if you plan on getting that, i think it will be tidious changing the battery packs just to use that extra slider